The invention relates to an electrosurgical device, in the form of a catheter, which is suitable for use in performing tissue ablation. More particularly, the invention features methods and devices to prevent excessive heating of the electrode.
The ablation of selected areas of organ tissue can be performed during surgical procedures to treat disease or medical disorders. Ablation of certain cardiac tissue is performed with increasing frequency to treat certain heart disorders that result in arrhythmia.
The heart is a muscular organ comprising four separate chambers which cooperate to pump blood throughout the body. The heart muscles must contract and relax in a coordinated sequence in order for blood to be passed through the circulatory system in an efficient manner. The heart includes a specialized system for generating impulses to cause rhythmical contraction of the heart muscle and for conducting these impulses rapidly through the heart. In the proper sequence the atria contract about one sixth of a second prior to ventricles. This enables extra filling of the ventricles before they contract to pump blood through the lungs and to other areas of the body.
The rhythmic impulse of the heart is generated in the sinoatrial node (SA node). The SA node has an inherent rhythm which can be modified by the sympathetic and parasympathetic nervous system. The impulse initiated by the SA node spreads through the atrium to the atrio-ventricular node (AV node), and then through the Purkinje fibers to the endocardial surfaces of the ventricles.
The rhythmical and conduction system of the heart is susceptible to disruption by disease. Damage caused to cardiac tissue can result in the inability of the cardiac conduction pathways to properly transmit the electrical impulses generated in the SA node, leading to arrhythmias, or irregular heartbeats. Cardiac arrhythmias can often be detected through electrocardiograms.
Some forms of cardiac arrhythmia are able to be controlled through medication. However, other forms of arrhythmia do not respond to medication. Moreover, medication typically does not cure the problem, and the dosage and the medication type must be changed periodically to enable continued control of the problem.
One alternative to medication is the surgical removal of a portion of the cardiac pathway which is responsible for the arrhythmia. The many dangers associated with open heart surgery render this a less preferred treatment option. Recently, however, it has become possible to intravascularly insert a specialized catheter within the heart, for positioning adjacent to the conduction tissue responsible for the arrhythmia. The catheter is adapted to deliver energy (e.g., radio frequency energy) to ablate or destroy the tissue responsible for an arrhythmia. This has been found to be a relatively safe and effective technique for eliminating many causes of arrhythmia. Various ablation catheters and techniques for their use are described in U.S. Pat. Nos. 4,641,649; 4,785,815; 4,869,248; and 4,896,671.
Cardiac ablation catheters typically have at least one electrode at the distal end of the catheter which is adapted to deliver energy to the tissue lesion. Other electrodes can be proximally positioned on the catheter and used for sensing endocardial signals. Ablation may be achieved by the application of electrical energy, such as radio frequency (RF) or direct current (DC) energy, from a generator source, through a conductor disposed within the catheter, and to the distal electrode.
During ablation procedures, energy, typically in the form of RF energy, is delivered to tissue by one or more electrodes mounted on an endocardial catheter. The delivery of the RF energy through the electrodes results in an associated temperature rise in the electrodes, and the heat is transferred to adjacent tissue. Although the application of heat to tissue can destroy the tissue (thus eliminating the arrhythmia), it is preferable to have the tissue ablation effected by the application of RF energy. Excess heating of the tissue can prolong the ablation procedure as the energy must be applied intermittently over a longer period of time to prevent an excessive rise in tissue temperature. Moreover, if thermal rather than electrical destruction of tissue is effected it is often not possible to achieve deeper penetration of the energy because the rise in tissue impedance in tissue adjacent the catheter inhibits the delivery of RF energy to deeper tissue. This is most commonly a problem where it is necessary to treat deeper or larger lesions.
It would thus be advantageous to develop an ablation catheter, suitable for use in cardiac ablation procedures, that is able to effectively deliver electrosurgical energy to tissue, without associated excessive heating of the ablation electrode and the adjacent tissue.
It is thus an object of the invention to provide a catheter suitable for use with cardiac ablation procedures utilizing the delivery of radio frequency energy. A further object is to provide an ablation catheter that more effectively delivers radio frequency energy to desired tissue without a significant transfer of heat to tissue from the electrode. Another object of the invention is to provide such an ablation catheter together with a system for controlling the temperature of ablation electrodes. It is also an object of the invention to provide an ablation catheter able to operate in a bipolar mode. Other objects will be apparent upon reading the disclosure which follows.